Precisely defining the requirements of specific amino acids in premature infants is important in order to provide appropriate nutrition to support normal growth. For example, although tyrosine and cysteine are considered non- essential amino acids in human adults, they are widely considered essential amino acids for premature infants, although some conflicting data exists. Whether tyrosine and cysteine are essential amino acids in premature infants is clinically important because currently available amino acid solutions used in parenteral nutrition contain neither of these amino acids in appreciable amounts. Furthermore, understanding the metabolism of tyrosine is particularly relevant for premature infants. Not only is it uncertain whether adequate amounts of tyrosine can be formed from phenylalanine (through hydroxylation), it is also unclear to what extent tyrosine can be appropriately catabolized (oxidized). Transient neonatal hypertyrosinemia, often observed in premature infants, is thought to be due to immaturity in the enzyme involved in tyrosine oxidation. Thus, a detailed examination of tyrosine metabolism in preterm infants is necessary in order to provide tyrosine in amounts sufficient for growth but which do not exceed the capacity for catabolism.